In strain wave gearings, a wave generator causes a flexible externally toothed gear to flex into an ellipsoidal shape and partially mesh with a rigid internally toothed gear, and rotation of the wave generator causes the meshing positions between the two gears to move in the circumferential direction. This generates relative rotation between the two gears that is commensurate with the difference in number of teeth between the two gears.
When one of the gears is secured so as not to rotate, the rotation inputted by the wave generator is outputted from the other gear as reduced rotation that is reduced in speed in accordance with the difference in number of teeth between the two gears. The wave generator, which causes the flexible externally toothed gear to flex into an ellipsoidal shape, typically comprises a metal rigid plug comprising an ellipsoidal outer-circumferential shape, and a wave bearing mounted on the ellipsoidal outer-circumferential shape.
Strain wave gearings of a hollow type are known in which a hollow section extends axially through the central portion of the strain wave gearing. In hollow strain wave gearings, a cylindrical wave plug comprising a hollow section that extends through the central portion thereof is used as the wave plug of the wave generator.
There is a demand for increased usage torque and increased hollow diameter in strain wave gearings. In cases where the usage torque is large, tooth-skipping (ratcheting) of the strain wave gearing is a concern when the hollow diameter is increased in size. In particular, in order to form a hollow section having a larger inner diameter in flat and top-hat-shaped strain wave gearings, it is necessary to form a larger hollow section in the wave plug. When a large hollow section is formed in the wave plug, the wall thickness thereof is reduced, and the radial rigidity thereof is reduced. As a result, the load torque at which tooth-skipping occurs between the two gears also decreases. A wave plug having high rigidity even when the cross-section (wall thickness, width) is limited is needed in order to avoid such circumstances.
Ceramics (alumina: 350 GPa, silicon nitride: 320 GPa) and carbon-fiber-reinforced plastics (also referred to below as “CFRP”) are known as materials that have a higher longitudinal modulus of elasticity than does the steel (longitudinal modulus of elasticity: 210 GPa) that is typically used as the material of the wave plug.
Carbon-fiber-reinforced plastic has a wide range of longitudinal elasticity coefficients, from 55 to 400 GPa, due to the modulus of elasticity of the carbon fibers used. Patent Document 1 discloses a material in which CFRP is used. In Patent Document 1, it is proposed that a material having a sandwich structure, in which CFRP layers are layered on the obverse and reverse of a vibration-damping plate, be used in a rectangular tubular frame for forming a fork in a robot hand.